As electric products are becoming lighter, thinner, shorter, smaller and more functional, which is typically shown in cellular phones, a printed circuit board used in such electric products is also becoming more precise. The multi layered printed circuit board has been developed in this trend, the board is manufactured by a so-called build-up method. The multi layered printed circuit board has a core board, on both back and front faces of which resin isolation layers and conductive patterns made of copper or the like are alternately formed and laminated. The layers are conductive through a hole called a via hole that is plated by copper. The conductive patterns are formed by a lithography method using an aligner having a photo mask that is depicted with an original pattern.
When building up the layers, it is very important to adjust the positions of the layers. The new pattern on a new build up layer must be precisely formed at a certain position relevant to the old pattern on the old layer formed already. To accomplish the alignment between the new and old patterns, an alignment mark depicted on the film mask and a board mark formed on the board are utilized.
Furthermore, the position of the via hole must be precisely determined in relation to the unseen pattern formed on a lower layer.
However, the alignment mark on the core board is invisible because the layer is covered by copper foil before the pattern is formed.